Optical coupling of light into touch-sensing systems

ABSTRACT

A touch-sensitive apparatus operates by light frustration (FTIR) and comprises a light transmissive panel ( 1 ) with a front surface ( 5 ) and a rear surface ( 6 ). Light emitters ( 2 ) and light detectors are optically coupled to the panel ( 1 ) to define a grid of light propagation paths inside the panel ( 1 ) between pairs of light emitters ( 2 ) and light detectors. A light in coupling structure comprises a diffusively reflective element ( 20 ) on the rear surface ( 6 ) and a specularly reflective element ( 22 ) on the front surface ( 5 ). Each light emitter ( 2 ) is arranged to project a beam of light onto a transmissive surface portion ( 24 ) on the rear surface ( 6 ), such that at least a portion of the beam of light enters the light transmissive panel ( 1 ) through the transmissive surface portion ( 24 ), is specularly reflected against the specularly reflective element ( 22 ) and impinges on the diffusively reflective element ( 20 ) from inside the light transmissive panel ( 1 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of Swedish patent application No. 1251437-8, filed 17 Dec. 2012, and U.S. provisional application No. 61/738,059, filed 17 Dec. 2012, both of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to touch-sensing systems that operate by propagating light by internal reflections along well-defined light paths inside a light transmissive panel, and in particular to optical solutions for coupling light into the light transmissive panel.

BACKGROUND ART

This type of touch-sensing system may be implemented to operate by transmitting light inside a thin light transmissive panel of solid material, which defines two parallel boundary surfaces connected by a peripheral edge surface. Light generated by a plurality of emitters is coupled into the panel so as to propagate by total internal reflection (TIR) between the boundary surfaces to a plurality of detectors. The light thereby defines propagation paths across the panel, between pairs of emitters and detectors. The emitters and detectors are arranged such that the propagation paths define a grid on the panel. An object that touches one of the boundary surfaces (“the touch surface”) will attenuate (“frustrate”) the light on one or more propagation paths and cause a change in the light received by one or more of the detectors. The location (coordinates), shape or area of the object may be determined by analyzing the received light at the detectors. This type of apparatus has an ability to detect plural objects in simultaneous contact with the touch surface, known as “multi-touch” in the art. Since light is frustrated by the touching object, this type apparatus is also known as an FTIR-based touch system (FTIR, Frustrated Total Internal Reflection).

In one configuration, e.g. disclosed in US2006/0114237, the light is coupled into the panel directly through the peripheral edge surface. Such an approach allows the light to be simply and efficiently injected into the panel. Also, such an incoupling does not add significantly to the thickness of the touch system. However, incoupling via the edge surface requires the edge surface to be highly planar and free of defects. This may be difficult and/or costly to achieve, especially if the panel is thin and/or manufactured of a comparatively brittle material such as glass. Incoupling via the edge surface may also add to the footprint of the touch system. Furthermore, it may be difficult to optically access the edge surface if the panel is attached to a mounting structure, such as a frame or bracket, and it is also likely that the mounting structure causes strain in the edge surface. Such strain together with load variations may result in undesirable variations in incoupling efficiency.

U.S. Pat. No. 3,673,327 discloses an FTIR-based touch system in which the emitters and detectors are arranged in rows on opposite ends of the panel, and light beams are propagated between opposite pairs of emitters and detectors so as to define a rectangular grid of propagation paths. Large prisms are attached to the bottom surface of the panel to couple the light beams into and out of the panel.

In U.S. Pat. No. 7,432,893, a few large emitters are arranged at the corners of the panel, or centrally on each side of the panel, to inject diverging light beams (“fan beams”) into the panel for receipt by linear arrays of photodiodes along all sides of the panel. Each fan beam is coupled into the panel by a large revolved prism which is attached to the top surface of the panel, and the photodiodes are attached to the top or bottom surface of the panel, so as to define a plurality of propagation paths between each prism and a set of photodiodes.

By attaching prisms or wedges to the top or bottom surfaces, it is possible to relax the surface requirements of the edge surface and/or to facilitate assembly of the touch system. However, the prisms or wedges may add significant thickness and weight to the system. To reduce weight and cost, the wedge may be made of plastic material. On the other hand, the panel is often made of glass, e.g. to attain required bulk material properties (e.g. index of refraction, transmission, homogeneity, isotropy, durability, stability, etc) and surface evenness of the top and bottom surfaces. The present applicant has found that the difference in thermal expansion between the plastic material and the glass may cause a bulky wedge to come loose from the panel as a result of temperature variations during operation of the touch system. Even a small or local detachment of the wedge may cause a significant decrease in the performance of the system.

In the field of LCD display technology, which is outside the field of touch-sensing systems, it is known to couple light from LEDs into thin light guide panels as part of so-called backlights (BLUs, Backlight units) for LCD displays. These light guide panels are located behind the LCD and are configured to emit light across its top surface to uniformly illuminate the rear side of the LCD. Various strategies for coupling light into light guide panels for the purpose of back-illuminating LCD displays are disclosed in the publication “Using micro-structures to couple light into thin light-guides”, by Yun Chen, Master of Science Thesis, Stockholm 2011, TRITA-ICT-EX-2011:112.

In the field of integrated optical sensors, which is outside the field of touch-sensing systems, it is also known to couple light into and out of a planar waveguide. In the article “Light coupling for integrated optical waveguide-based sensors”, by Steindorfer et al., published in Optical Sensing and Detection, proceedings of the SPIE, vol. 7726, pp. 77261S-1-77261S-10 (2010), an optical waveguide is deposited on the upper side of a substrate to be exposed to an analyte. An organic light emitting diode (OLED), which acts as a light source, and an organic photodiode as light detector are monolithically integrated on the lower side of the substrate. Fluorescent molecules are deposited on the upper side, to couple light emitted by the OLED into the waveguide, and a scattering layer is applied to the upper side to couple light out of the waveguide onto the photodiode.

SUMMARY

It is an objective of the invention to at least partly overcome one or more of limitations of prior art FTIR-based touch systems.

One objective is to provide a touch-sensitive apparatus which is compact, while defining light propagation paths with well-defined extent.

Another objective is to enable a touch-sensitive apparatus with restricted external access to the edge surface.

Yet another objective is to enable a touch-sensitive apparatus that is simple to assemble and suited for mass production.

One or more of these objectives, as well as further objectives that may appear from the description below, are at least partly achieved by a touch-sensitive apparatus according to the independent claims, embodiments thereof being defined by the dependent claims.

A first aspect of the invention is a touch-sensitive apparatus, which comprises: a light transmissive panel that defines a front surface and an opposite, rear surface; a plurality of light emitters optically coupled to the light transmissive panel and a plurality of light detectors optically coupled to the light transmissive panel, so as to define a grid of light propagation paths inside the light transmissive panel between pairs of light emitters and light detectors; wherein a structure for optically coupling one of the light emitters to the light transmissive panel comprises a first reflective element on the rear surface and a second reflective element on the front surface, the first reflective element being configured to be diffusively reflective to impinging light from inside the light transmissive panel, and the second reflective element being configured to be specularly reflective to impinging light from inside the light transmissive panel; and wherein said one light emitter is arranged to project a beam of light onto a transmissive surface portion on the rear surface, such that at least a portion of the beam of light enters the light transmissive panel through the transmissive surface portion, is specularly reflected against the second reflective element and impinges on the first reflective element from inside the light transmissive panel.

In this touch-sensitive apparatus, the first reflective element will act as a secondary light source which is located in contact with the panel to emit diffuse light into the panel. This secondary light source defines the actual origin of the propagation path(s) that are generated by the light from the light emitter. Thus, origin of the propagation path(s) is given by the first reflective element, which may have a well-defined location and extent on the rear surface. Further, the first reflective element is diffusively reflective and thereby re-distributes the incoming light more or less randomly. This means that the first reflective element has the ability to act as a secondary light source for many different types of light emitters and is relatively insensitive to manufacturing and mounting tolerances for the light emitter. The first and second reflective elements are also simple to apply to the panel. All in all, this facilitates mass production.

The combination of the first and second reflective elements also enables a compact and light-weight touch-sensitive apparatus, since one or both of the first and second reflective elements may be provided as sheet-like elements.

Further, the incoupling structure allows light to be coupled into the panel also with restricted external access to the edge surface, since the transmissive surface portion is located on the rear surface of the panel.

The touch-sensitive apparatus allows the light emitter to be arranged underneath the panel, which may reduce the footprint of the apparatus.

In one embodiment, the transmissive surface portion is defined within the extent of the first reflective element. For example, the first reflective element may be shaped as a ring of diffusively reflective material surrounding the transmissive surface portion.

In an alternative embodiment, the transmissive surface portion is formed by an uncovered portion of the rear surface between the first reflective element and a peripheral edge of the panel.

In one embodiment, the structure is configured to define an origin for a subset of the light propagation paths, said subset of light propagation paths extending in different directions from the structure across the light transmissive panel, and wherein the first reflective element is configured to have different extent in at least part of said different directions, and wherein the extent of the first reflective element in a subset of the different directions exceeds a limit extent W_(lim)=2·t·tan(θ_(min)), with t being a thickness of the light transmissive panel and θ_(min) being a minimum angle of incidence for light to propagate by internal reflections inside the light transmissive panel. In one example, the first reflective element is elliptical.

In one embodiment, the light transmissive panel comprises a peripheral edge surface that connects the front and rear surfaces, wherein a third reflective element is arranged on the peripheral edge surface adjacent to the first and second reflective elements, and wherein the third reflective element is configured to be diffusively reflective to impinging light from inside the light transmissive panel.

In one embodiment, the first, second and third reflective elements define at least part of a reflective enclosure that extends from the first reflective element across the peripheral edge surface to the second reflective element.

In one embodiment, said one light emitter is arranged to project the beam of light onto the transmissive surface portion such that a portion of the beam of light impinges on the third reflective element.

In one embodiment, the first reflective element is a sheet-like element applied to or integrated in the rear surface of the light transmissive panel.

In one embodiment, the first reflective element is non-transmissive to the beam of light.

In one embodiment, the second reflective element is arranged opposite to the first reflective element.

In one embodiment, the first reflective element is configured to exhibit at least 50% diffuse reflection, and preferably at least 90% diffuse reflection.

In one embodiment, the first reflective element is a near-Lambertian diffuser.

In one embodiment, the first reflective element is configured to promote emission of diffusively reflected light at angles that sustain light propagation by total internal reflection inside the light transmissive panel.

In one embodiment, the light emitter is optically coupled to the transmissive surface portion by a light transmissive material with a refractive index that is adapted or matched to the refractive index of the light transmissive panel at the transmissive surface portion.

In one embodiment, the first reflective element is configured as an elongate strip, and the second reflective element is configured as an elongate strip which is arranged on the front surface to co-extend with the first reflective element, wherein the first and second reflective elements define a structure for coupling light from a number of light emitters into the light transmissive panel, the number of light emitters being located beneath the light transmissive panel dispersed along the first reflective element. The first and second reflective elements may define a respective frame structure around a center portion of the light transmissive panel.

In an alternative embodiment, the touch-sensitive apparatus comprises a plurality of first reflective elements which are spatially separated and arranged on the rear surface along a rim of the light transmissive panel, wherein each of the first reflective elements is included in a structure for optically coupling a number of light emitters to the light transmissive panel, said number of light emitters being located beneath the light transmissive panel dispersed along the rim of the light transmissive panel.

In one embodiment, the second reflective element is configured to block light that is visible to the human eye.

A second aspect of the invention is a touch-sensitive apparatus, which comprises: a light transmissive panel that defines a front surface and an opposite, rear surface, and a peripheral edge surface that connects the front and rear surfaces; a plurality of light emitters optically coupled to the light transmissive panel and a plurality of light detectors optically coupled to the light transmissive panel, so as to define a grid of light propagation paths inside the light transmissive panel between pairs of light emitters and light detectors; wherein a structure for optically coupling one of the light emitters to the light transmissive panel comprises a reflective element on the peripheral edge surface, the reflective element being configured to be diffusively reflective to impinging light from inside the light transmissive panel, and wherein said one light emitter is arranged to project a beam of light onto the rear surface, such that at least a portion of the beam of light enters the light transmissive panel and at least partly impinges on the reflective element from inside the light transmissive panel. The second aspect provides similar technical advantages as the first aspect.

Any one of the above-identified embodiments of the first aspect may be adapted and implemented as an embodiment of the second aspect.

Still other objectives, features, aspects and advantages of the present invention will appear from the following detailed description, from the attached claims as well as from the drawings.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described in more detail with reference to the accompanying schematic drawings.

FIG. 1 is a section view of a light transmissive panel to illustrate the principle of using FTIR for touch detection.

FIG. 2 is a top plan view of an FTIR-based touch-sensitive apparatus according to an embodiment.

FIG. 3 is a 3D plot of an attenuation pattern generated based on energy signals from an FTIR-based touch-sensitive apparatus.

FIGS. 4A-4B are section views of embodiments of light incoupling structures in a touch-sensitive apparatus.

FIG. 5A is a section view of a light incoupling structure with non-perpendicular emission of diffusely reflected light, and FIG. 5B is a section view of an embodiment with combined diffusive and direct incoupling of light.

FIGS. 6A-6D are bottom plan views of exemplary diffusers.

FIGS. 7-8 are section views of alternative light incoupling structures.

FIGS. 9A-9B are bottom plan views of exemplary diffusers.

FIGS. 10A-10B are section views of further embodiments.

FIG. 11 is a bottom plan view of a strip-based light incoupling structure.

FIG. 12A is a top plan view of a touch-sensitive apparatus with a strip-based structure for light incoupling and light outcoupling, and FIG. 12B is a top plan view of a touch-sensitive apparatus with individual diffusers for light incoupling and light outcoupling.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following, various inventive light incoupling structures will be presented as installed in an exemplifying FTIR-based touch-sensitive apparatus. Throughout the description, the same reference numerals are used to identify corresponding elements.

FIG. 1 illustrates the concept of touch detection based on attenuation of propagating light, commonly denoted FTIR (Frustrated Total Internal Reflection). According to this concept, light is transmitted inside a panel 1 along a plurality of well-defined propagation paths. The panel 1 is made of solid material in one or more layers and may have any shape. The panel 1 defines an internal radiation propagation channel, in which light propagates by internal reflections. In the example of FIG. 1, the propagation channel is defined between the boundary surfaces 5, 6 of the panel 1, and the top surface 5 allows the propagating light to interact with touching objects 7 and thereby defines the touch surface 4. The interaction is enabled by injecting the light into the panel 1 such that the light is reflected by total internal reflection (TIR) in the top or front surface 5 as it propagates through the panel 1. The light may be reflected by TIR in the bottom or rear surface 6 or against a reflective coating thereon. It is also conceivable that the propagation channel is spaced from the bottom surface 6, e.g. if the panel comprises multiple layers of different materials. The panel 1 may thus be made of any solid material (or combination of materials) that transmits a sufficient amount of light in the relevant wavelength range to permit a sensible measurement of transmitted energy. Such material includes glass, poly(methyl methacrylate) (PMMA) and polycarbonates (PC). The panel 1 typically has a refractive index in the range of 1.3-1.7. The panel 1 may be designed to be overlaid on or integrated into a display device or monitor (not shown).

As shown in FIG. 1, an object 7 that is brought into close vicinity of, or in contact with, the touch surface 4 may interact with the propagating light at the point of touch. In this interaction, part of the light may be scattered by the object 7, part of the light may be absorbed by the object 7, and part of the light may continue to propagate in its original direction across the panel 1. Thus, the touching object 7 causes a local attenuation or “frustration” of the total internal reflection, which leads to a decrease in the energy (or equivalently, the power or intensity) of the transmitted light, as indicated by the thinned lines downstream of the touching objects 7 in FIG. 1.

FIG. 2 illustrates an example embodiment of a touch-sensitive apparatus 100 that is based on the concept of FTIR. Emitters 2 are distributed along the perimeter of the touch surface 4, beneath the panel 1, to project light onto an incoupling structure on the panel 1 such that at least part of the light is captured inside the panel 1 for propagation by internal reflections in the propagation channel. Detectors 3 are distributed along the perimeter of the touch surface 4 and are optically coupled to the panel 1 so as to receive part of the propagating light. The light from each emitter 2 will thereby propagate inside the panel 1 to a number of different detectors 3 on a plurality of light propagation paths D. Even if the light propagation paths D correspond to light that propagates by internal reflections inside the panel 1, the light propagation paths D may conceptually be represented as “detection lines” that extend across the touch surface 4 between pairs of emitters 2 and detectors 3, as indicated by dotted lines in FIG. 2. Thus, the detection lines correspond to a projection of the propagation paths onto the touch surface 4. Thereby, the emitters 2 and detectors 3 collectively define a grid of detection lines (“detection grid”) on the touch surface 4, as seen in a top plan view. It is appreciated that FIG. 2 is an example, and that (significantly) larger number of emitters 2 and/or detectors 3 may be included in the apparatus 100. Also, the distribution of emitters 2 and detectors 3 may differ.

As used herein, the emitter 2 may be any type of device capable of emitting radiation in a desired wavelength range, for example a diode laser, a VCSEL (vertical-cavity surface-emitting laser), an LED (light-emitting diode), an incandescent lamp, a halogen lamp, etc. The emitter 2 may also be formed by the end of an optical fiber. The emitters 2 may generate light in any wavelength range. The following examples presume that the light is generated in the infrared (IR), i.e. at wavelengths above about 750 nm. Analogously, the detector 3 may be any device capable of converting light (in the same wavelength range) into an electrical signal, such as a photo-detector, a CCD device, a CMOS device, etc.

The detectors 3 collectively provide an output signal, which is received and sampled by a signal processor 10. The output signal contains a number of sub-signals, also denoted “projection signals”, each representing the energy of light received by a certain light detector 3 from a certain light emitter 2. Depending on implementation, the signal processor 10 may need to process the output signal for separation of the individual projection signals. The projection signals represent the energy, intensity or power of light received by the detectors 3 on the individual detection lines D. Whenever an object touches a detection line, the received energy on this detection line is decreased or “attenuated”.

The signal processor 10 may be configured to process the projection signals so as to determine a property of the touching objects, such as a position (e.g. in the x,y coordinate system shown in FIG. 2), a shape, or an area. This determination may involve a straight-forward triangulation based on the attenuated detection lines, e.g. as disclosed in U.S. Pat. No. 7,432,893 and WO2010/015408, or a more advanced processing to recreate a distribution of attenuation values (for simplicity, referred to as an “attenuation pattern”) across the touch surface 1, where each attenuation value represents a local degree of light attenuation. An example of such an attenuation pattern is given in the 3D plot of FIG. 3, where the peaks of increased attenuation represent touching objects. The attenuation pattern may be further processed by the signal processor 10 or by a separate device (not shown) for determination of a position, shape or area of touching objects. The attenuation pattern may be generated e.g. by any available algorithm for image reconstruction based on projection signal values, including tomographic reconstruction methods such as Filtered Back Projection, FFT-based algorithms, ART (Algebraic Reconstruction Technique), SART (Simultaneous Algebraic Reconstruction Technique), etc. Alternatively, the attenuation pattern may be generated by adapting one or more basis functions and/or by statistical methods such as Bayesian inversion. Examples of such reconstruction functions designed for use in touch determination are found in WO2009/077962, WO2011/049511, WO2011/139213, WO2012/050510, and WO2013/062471, all of which are incorporated herein by reference.

In the illustrated example, the apparatus 100 also includes a controller 12 which is connected to selectively control the activation of the emitters 2 and, possibly, the readout of data from the detectors 3. Depending on implementation, the emitters 2 and/or detectors 3 may be activated in sequence or concurrently, e.g. as disclosed in WO2010/064983. The signal processor 10 and the controller 12 may be configured as separate units, or they may be incorporated in a single unit. One or both of the signal processor 10 and the controller 12 may be at least partially implemented by software executed by a processing unit 14, such as a CPU.

The structure for coupling light into the panel in FIG. 2 will now be explained in more detail with reference to FIG. 4A, which illustrates a section of the touch-sensitive apparatus along any one of the detection lines D in FIG. 2, from the incoupling structure into the touch surface 4. Before looking at the light incoupling structure in FIG. 4A, a few definitions will be given. As used herein, “specular reflection” is given its ordinary meaning, which refers to the mirror-like reflection of light from a surface, in which light from a single incoming direction (a ray) is reflected into a single outgoing direction. Specular reflection is described by the law of reflection, which states that the direction of incoming light (the incident ray), and the direction of outgoing light reflected (the reflected ray) make the same angle with respect to the surface normal, and that the incident, normal, and reflected directions are coplanar. As used herein, “diffuse reflection” is given its ordinary meaning, which refers to reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in specular reflection. The diffuse reflection is also known as “scattering”. The skilled person appreciates that many surfaces/elements/materials exhibit a combination of specular and diffuse reflection. As used herein, a surface is considered “diffusively reflective” when at least 20% of the reflected light is diffuse. The relation between diffuse and specular reflection is a measurable property of all surfaces/elements/materials.

In FIG. 4A, the incoupling structure is defined by a combination of a diffusively reflective element 20, denoted “diffuser” in the following, and a specularly reflective element 22, denoted “reflector” in the following. The diffuser 20 is arranged on the rear surface 6 at the rim of the panel 1, and the reflector 22 is arranged on the front surface 5, opposite to the diffuser 20. The diffuser 20 defines within its extent a light transmissive surface or window 24. The window 24 may be a cut-out or hole in the diffuser 20, as shown, or it may be a light transmissive material integrated into the diffuser 20. An emitter 2 is arranged beneath the panel 1 to optically face the window 24. The emitter 2 is operable to emit an expanding (diverging) beam of light onto the window 24. In FIG. 4A, the emitter 2 is mounted on a connecting substrate 26 such as a PCB (Printed Circuit Board) which is designed to supply power and transmit control signals to the emitter 2.

The emitter 2 is placed such that (part of) the emitted beam of light is transmitted into the panel 1 through the window 24 and forms a diverging cone of light in the panel 1. The emitter 2 and the window 24 are arranged such that the diverging cone of light strikes the reflector 22. In FIG. 4A, the angle α indicates the divergence of the light that hits the window 24 and the angle β indicates the divergence of the light that has entered the panel. It is understood that the emitter 2 may have a larger divergence, such that the emitted beam illuminates an area larger than the window 24. This may reduce the impact of mounting tolerances on the incoupling efficiency.

As indicated in FIG. 4A, the cone of light strikes the reflector 22 and is specularly reflected back towards the rear surface 6 where it strikes the diffuser 20. The incoming rays will be diffusively reflected by the diffuser 20, causing emission of diffuse light in a large solid angle inside the panel 1, as indicated in FIG. 4A by encircled rays 30. It should be noted that the diffuse light is emitted omnidirectionally from the diffuser 20 as seen in plan view (cf. FIG. 2). The portion of the diffusively reflected light that hits the front surface 5 at an angle equal to or larger than a minimum propagation angle θ_(min) will propagate by TIR in the propagation channel inside the panel 1, as indicated by the rays extending away from the encircled rays 30 in FIG. 4A. Light that hits the front surface 5 at an angle below the minimum propagation angle θ_(min) will escape the propagation channel. The minimum propagation angle θ_(min) is typically given by the largest critical angle of the propagation channel. The critical angle θ_(c) is given by Snell's law and is well known to the skilled person. The largest critical angle is set by the smallest difference in refractive index across the boundary surfaces that define the propagation channel.

Both the diffuser 20 and the reflector 22 are preferably non-transmissive to the light from the emitter 2, to avoid that light escapes the panel 1 through the diffuser 20 or the reflector 22.

It should be noted that some diffusively reflected light will impinge on the reflector 22 and be specularly reflected back onto the diffuser 20, and result in further diffusively reflected light. Thus, the incoupling structure in FIG. 4A provides an inherent “recycling” of available light that will improve coupling efficiency. This recycling ability may be increased by increasing the extent of the reflector 20.

In the incoupling structure of FIG. 4A, the diffuser 20 will act as a secondary light source which is located in contact with the propagation channel inside the panel 1 to emit diffuse light. The secondary light source thereby defines the actual origin of the detection lines that are generated by the light from the respective emitter 2. Since the diffuser 20 more or less randomly re-distributes the incoming light, the importance of the luminance profile of the emitter 2 is reduced or even eliminated. This means that the diffuser 20 has the ability to act as a secondary light source for many different types of emitters 2, as long as the light from the emitter 2 hits the diffuser 20, via the reflector 20, with a proper extent and at a proper location. The use of the diffuser 20 enables a compact and light-weight configuration of the apparatus 100. For example, the diffuser 20 may be provided as a sheet-like element attached to or integrated in the rear surface 6. Such a sheet-like element may be so thin and flexible that it is able to absorb shear forces that may occur in the interface between the diffuser 20 and the panel 1, e.g. caused by differences in thermal expansion as discussed in the Background section. Furthermore, since the emitter 2 is arranged to optically face the panel 1, the PCB 26 (if present) may be arranged flat along the rear surface 6. Still further, the reflector 22 may be implemented as a coating or layer of specularly reflective material, which may be so thin that it does not obstruct the user during interaction with the touch surface 4. Furthermore, the incoupling structure allows light to be coupled into the panel 1 irrespective of the quality and design of the edge surface 10.

As indicated in FIG. 4A, the emitter 2 may be located with an air gap to the panel 1. This may facilitate assembly of apparatus 100. However, the provision of an air gap will restrict the divergence β of the light that enters the panel 1 to angles that do not sustain propagation by TIR, i.e. β/2<θ_(min). In other words, the entering light can only be coupled into the panel 1 for propagation by TIR by reflection onto the diffuser 20 (“diffusive coupling”).

In certain implementations, it may be desirable to use a combination of diffusive coupling and “direct coupling”, i.e. to allow certain parts of the emitted beam to directly strike the front surface 5 at an angle above the minimum propagation angle θ_(min), i.e. to allow β/2>θ_(min). This may be achieved by placing a spacer 28 of solid light transmissive material intermediate the emitter 2 and the panel 1, as shown in FIG. 4B, where the thickness and index of refraction of the spacer 28 is selected so as to provide an adequate divergence β of the entering light. The light that enters the panel in FIG. 4B will thus undergo different processes: a first part (between dashed arrows) will hit the diffuser 20 via the reflector 20 and result in diffusively reflected light (some of which will be coupled into the panel 1 for propagation by TIR), a second part (between the dotted arrow and the right-hand solid arrow) will be directly coupled into the panel 1 for propagation by TIR, and the rest of the light will be transmitted out of the panel 1 through the front surface 5 and the edge surface 10.

Examples of suitable spacer materials include optically clear glue, gel and silicon. Even if not shown in FIG. 4B, the light from the emitter 2 may be refracted when passing into the spacer 28 from the emitter 2, e.g. if the spacer 28 has a similar (“matched”) index of refraction as the panel 1 and a larger index of refraction than the emitter 2. Generally, the use of a spacer 28 may increase the coupling efficiency, for a given size of the window 24, if the refraction caused by the spacer 28 operates to increase the effective amount of light that hits the window 24 and is transmitted into the panel 1. The use of a spacer 28 may also serve to reduce reflection losses compared to the use of an air gap. A spacer 28 is included in all of the following embodiments, although an air gap may be used instead.

In all embodiments, the reflector 22 may be made of any specularly reflective material, e.g. a metal such as aluminum, copper or silver, or a multilayer structure, as is well-known to the skilled person. A protective coating (not shown) may be applied onto the reflector 22 to protect it from scratches, abrasion etc.

In all embodiments, the diffuser 20 may be selected or configured to provide a given divergence of the diffusively reflected light 30 (defined by the total angle between the off-axis angles where the luminous intensity is 50% of the on-axis value). If the distribution of the diffusively reflected light 30 has a main direction which is normal to the diffuser 20, as shown in FIG. 4A, the divergence needs to be large enough so that a portion of the diffusively reflected light 30 hits the front surface 5 at an angle above the minimum propagation angle θ_(min). This typically means that the divergence should be at least 80° in all directions in the plane of the panel, and preferably at least 90°, 100° or 110°. In one embodiment, the diffuser 20 is near-Lambertian and has a divergence of approximately 120° in all directions in the plane of the panel.

The diffuser 20 may be implemented as a coating, layer or film applied to the rear surface 6. In one embodiment, the diffuser 20 is implemented as matte white paint or ink applied to the rear surface 6. In order to achieve a high diffuse reflectivity, it may be preferable for the paint/ink to contain pigments with high refractive index. One such pigment is TiO₂, which has a refractive index n=2.5-2.7. It may also be desirable, e.g. to reduce Fresnel losses, for the refractive index of the paint binder (vehicle) to match the refractive index of the surface material in the top surface. For example, depending on refractive index, a range of vehicles are available such as oxidizing soya alkyds, tung oil, acrylic resin, vinyl resin and polyvinyl acetate resin. The properties of the paint may be further improved by use of e.g. EVOQUE™ Pre-Composite Polymer Technology provided by the Dow Chemical Company. There are many other coating materials for use as a diffuser that are commercially available, e.g. the fluoropolymer Spectralon, polyurethane enamel, barium-sulphate-based paints or solutions, granular PTFE, microporous polyester, Makrofol® polycarbonate films, GORE® Diffuse Reflector Product, etc. Also, white paper may be used as diffuser 20.

Alternatively, the diffuser 20 may be implemented as a so-called engineered diffuser, which is attached to the rear surface 6 by an adhesive. Examples of engineered diffusers include holographic diffusers, such as so-called LSD films provided by the company Luminit LLC. According to other alternatives, the diffuser 20 may be implemented as a micro-structure in or on the rear surface 6 with an overlying coating of reflective material. The micro-structure may e.g. be provided in the rear surface 6 by etching, embossing, molding, abrasive blasting, etc.

As noted above, the diffuser 20 may exhibit a combination of diffuse and specular reflection. In the set up of FIGS. 4A-4B, any light that is specularly reflected by the diffuser 20 is likely to leave the panel 1 through the front surface 5 and result in coupling losses. It is thus preferred that the relation between diffusive and specular reflection is high for the diffuser 20. It is currently believed that reasonable performance may be achieved, at least for smaller touch surfaces, when the diffuser exhibits at least 50% diffuse reflection, i.e. when at least 50% of the reflected light is diffusively reflected. Preferably, the diffuser 20 is designed to reflect incoming light such that at least about 60%, 70%, 80%, 90%, 95%, or 99% of the reflected light is diffusively reflected.

FIG. 5A illustrates a variant in which the diffuser 20 is configured to emit the diffusively reflected light 30 with a distribution that is angled towards the touch surface 4, where the angle of the distribution is dependent of the angle of the impinging light on the diffuser 20. This variant may improve incoupling efficiency, since the diffuser 20 will promote diffuse reflection to angles that are capable of sustaining total internal reflection in the radiation propagation channel inside the panel 1. As shown in FIG. 5A, the diffuser 20 may be implemented as a combination of a transmissive diffuser layer 51 and an underlying specularly reflecting layer 52. The transmissive diffuser layer 51 may be optimized, e.g. with respect to its thickness, structure, etc, to achieve a high “diffusive transmission ratio”, i.e. a high ratio of diffusively transmitted light to diffusively reflected light. Generally, a high diffusive transmission ratio will cause the diffusively transmitted light to be more or less aligned with the main direction of the incoming light. The diffusively transmitted light will be specularly reflected by layer 52 and again transmitted by diffuser layer 51 into the panel 1. In a variant, the diffuser 20 in FIG. 5A may be configured with a diffractive structure that provides the angled distribution. In another variant, the angled distribution is achieved by a structured paint, i.e. a paint that includes asymmetric particles with a proper alignment to produce the angled distribution of diffusively reflected light 30. With an angled distribution, it may be sufficient for the divergence of the diffusively reflected light 30 to be at least 40°, 50° or 60°, although a larger divergence might be preferable.

FIG. 5B illustrates an embodiment which enables a combination of diffusive and direct coupling. Unlike the embodiment in FIG. 4B, the incoupling structure in FIG. 5B does not require β/2>θ_(min) to achieve direct coupling (although such a divergence may be used also in the embodiment of FIG. 5B). The direct coupling is achieved by a re-directing structure 53 which is attached to the rear surface 6 to receive, via the reflector 22, the light that enters the panel 1 in a general direction away from the touch surface 4. The structure 53 is arranged on the rear surface 6 between the window 24 and the edge surface 10 of the panel 1 and is configured to reflect (preferably specularly) at least a portion of the incoming light towards the touch surface 4 at an angle that exceeds θ_(min). As shown, the structure 53 may comprise micro-structured prisms with a respective reflection surface angled to provide the desired re-direction towards the touch surface 4. A diffuser 20 is arranged on the other side of the window 24 to diffusively reflect the light that enters the panel 1 in a general direction towards from the touch surface 4.

The diffuser 20 and the window 24 may have various shapes (contours) and extents. In certain embodiments, e.g. as shown in FIG. 4A, the extent of the diffuser 20 approximately matches the projected spot of the light from the emitter 2 onto the diffuser 20, i.e. after transmission through window 24 and specular reflection in reflector 22. However, the extent of the diffuser 20 may be larger, and the extent may be optimized by simulation, e.g. taking into account the divergence β and the direction of the entering light, the thickness of the panel, etc.

FIGS. 6A-6D show examples of diffusers 20 as applied to the rear surface 6 of the panel 1. In the examples, the diffusers 20 are configured as an annulus, i.e. a ring-shaped structure, with the window 24 at the center. In FIG. 6A, both the diffuser 20 and the window 24 have a circular extent. In FIG. 6B, the diffuser 20 and the window 24 have a substantially rectangular extent (with rounded corners). Other shapes are of course conceivable. However, the design in FIG. 6A has the advantage of providing equal extent of the diffuser 20 in all directions in the plane of the panel from the window 24, i.e. for all detection lines (cf. FIG. 2). This may serve to evenly distribute the light in the panel.

In other embodiments, it may be desirable to vary the distribution of light between different detection lines. This may be achieved by using a diffuser 20 with a non-circular contour that has an extent, in one or more directions in the plane of the panel, that exceeds the minimum distance between consecutive bounces in the rear surface 6. This “minimum bounce distance” is given by W_(lim)=2·t·tan(θ_(min)), with t being the thickness of the panel 1. In directions where the extent exceeds W_(lim), a portion of the diffusively reflected light that would have propagated in the propagation channel (i.e. with an angle of θ_(min) or there above) will instead impinge on the diffuser 20 and cause further diffusively reflected light. This means that the energy of the propagating light will decrease in these directions, compared to other directions where the extent does not exceed W_(lim). It is also to be noted that a non-circular diffuser will also cause the apparent extent of the origin of the propagating light to vary with direction across the touch surface, i.e. for different detection lines. One example of such a non-circular extent is given in FIG. 6C, where the diffuser 20 is elliptical with a major axis u1 and a minor axis u2. The dashed circle 60 indicates W_(lim). Based on the foregoing explanation, it is appreciated that the injected energy is higher for a detection line that extends along the minor direction u2 than for a detection line that extends along the major direction u1. Further, the light propagating in the minor direction u2 from the diffuser 20 is wider (in the plane of the panel) than the light propagating along the major direction u1 from the diffuser 20. FIG. 6D illustrates an elliptical diffuser 20 arranged on the rear surface 6 with a given rotation with respect to a set of detection lines D (the dotted lines indicate the main directions of the detection lines), to achieve a given distribution of light between the detection lines D. It is understood that the diffuser 20 may have a more complex contour and extent to achieve desired properties (energy and/or width) of each individual detection line D.

The effect of increasing the injected energy for certain detection lines (and decreasing the injected energy for other detection lines) may additionally or alternatively be achieved by tilting the main direction of the beam towards the touch surface 4 (cf. FIGS. 10A-10B), such that the main direction of the beam has a desired direction as seen in plan view of the panel 1.

The reflector 22 has an extent that at least matches, and preferably exceeds, the projection of the light from the emitter 2 onto the front surface 5. In all embodiments given herein, the reflector 22 may extend further towards the touch surface 4 (i.e. to the right in FIG. 4A), e.g. at least so that the inner end of the reflector 22 is vertically aligned with the inner end of the diffuser 20. It should be understood that the reflector 22 has been omitted in FIG. 2 and FIGS. 6A-6D.

The reflector 22 may be configured to provide the additional function of hiding the diffuser 22 from view through the panel 1, i.e. the reflector 22 may be non-transmissive to visible light. In a variant, another coating is applied onto the reflector 22 and selected parts of the front surface 5 to provide this functionality.

FIG. 7 illustrates an incoupling structure designed to further improve the incoupling efficiency. To this end, the reflector 22 extends to the top edge of the panel 1, a diffusively reflective coating 20A is applied to the edge surface 10, and a specularly reflective coating 22A is applied to the rear surface 6 between the bottom edge of the panel 1 and the outer end of the diffuser 20, so as to form a reflective enclosure at the rim of the panel 1. The same material may be used for the coating 22A and the reflector 22, and for the coating 20A and the diffuser 20, respectively. The reflective enclosure serves to increase the “recycling” of diffusively reflected light, specifically diffuse light that leaves the diffuser 20 in a direction away from the touch surface 4 (to the left in FIG. 7). This light will be at least partially reflected back towards the diffuser 20 by the reflective enclosure. The coating 22A may be provided to promote that the diffusively reflected light is generated closer to the touch surface 4, i.e. to right in FIG. 7. However, in a variant, the coating 22A is replaced by a diffusively reflective element.

In an alternative embodiment, not shown, the coating 20A is replaced by a coating that absorbs the light emitted by emitters 2. Suitable light absorbing materials include black paint and black chrome. Such an embodiment may be preferable when the distance between the window 24 and the edge surface 10 is excessive, e.g. at least equal to the minimum bounce distance W_(lim). At such excessive distances, the diffusively reflective coating 20A might form an additional light source in the propagation channel, causing difficulties in the reconstruction processing.

It should be noted that the window 24 need not be arranged at the center of the diffuser 20, as shown in FIGS. 6A-6D, but might be displaced from the center, e.g. towards the edge surface 10. One such embodiment is illustrated in FIG. 8, where the window 24 is formed between the bottom edge of the panel 1 and the outer end of the diffuser 20. Thereby, the light from the emitter is reflected, by the reflector 22, onto a diffuser 20 with a coherent surface of diffusively reflective material (i.e. without an included light transmissive opening). Thereby, the diffuser 20 enables diffuse reflection within its entire perimeter, which might serve to improve the light distribution inside the panel 1. In the illustrated embodiment, a reflective enclosure is provided by the reflector 22 and coatings 20A, 22A, like in FIG. 7.

FIGS. 9A-9B illustrate, in bottom plan views towards the rear surface 6, examples of how the window 24 may be defined outside the extent of the diffuser 20. In FIG. 9A, the diffuser 20 is a rectangle and the window 24 is formed by a non-coated portion of the rear surface 6 intermediate the outer end of the diffuser 20 and the bottom edge of panel 1. For clarity, the location of the emitter 2 is indicated by dashed lines. In FIG. 9B, the diffuser 20 is semi-circular and the window is at least partially formed by a semi-circular cut-out in the outer perimeter of the diffuser 20. It should be noted that the contour and extent of the diffuser 20 may be adapted to achieve desired properties (energy and/or width) of different detection lines, using the design principles described above in relation to FIGS. 6C-6D.

FIG. 10A is a section view of an apparatus 100 in which the emitter 2 is arranged to emit the diverging beam at an angle to the rear surface 6, such that the main direction (center line C) of the beam is tilted towards the touch surface 4. This may increase coupling efficiency. The tilted beam may be achieved by tilting the PCB 26 with the emitter 2 or, as shown, by using an emitter 2 which is configured to emit a beam of light with a main direction that is non-parallel to the normal of the PCB 26 when the emitter 2 is mounted on the PCB 26. Although not shown in FIG. 10A, the beam may be tilted to such an extent to achieve combined diffusive and direct incoupling of light, provided that the extent W of the diffuser 20 along the detection line is less than the minimum bounce distance W_(lim).

FIG. 10B is a section view of an alternative embodiment, in which the main direction (center line C) of the beam is tilted towards the edge surface 10. This embodiment operates by diffusive coupling, where part of the light is reflected by reflector 22 onto diffuser 20, part of the light is reflected by reflector 22 onto diffuser coating 20A, and part of the light directly strikes the diffuser coating 20A. The embodiment may better balance the contributions from the diffuser 20 and the diffuser coating 20A to the light that is coupled into the propagation channel.

In a variant of FIG. 10B, the diffuser 20, the coating 22A and the reflector 22 are omitted, such that light is coupled into the propagation channel by diffusive reflection on the diffuser coating 20A only. The emitted beam is preferably tilted such that it illuminates the entire diffuser coating 20A, and the beam may be directed with its center line C essentially centered on the diffuser coating 20A.

FIG. 11 is a bottom plan view of another embodiment. Here, the diffuser 20 is implemented as an elongate strip instead of a sequence of spatially separated diffusers (FIG. 2). In the illustrated embodiment, the diffuser strip 20 is provided with windows 24 within its extent. Although not shown, it is appreciated that each window 24 is configured to receive and transmit light from a respective emitter 2 towards the reflector 22, which is also implemented as an elongate strip opposite to the diffuser strip 20.

FIG. 12A is a top plan view of another embodiment, in which the emitters 2 (open circles) and detectors 3 (squares) are alternated along the rim of the panel 1. Diffusers 20 in the form of elongate strips are arranged on the rear surface 6 in a frame structure around a central portion 40 of the panel 1. Each diffuser strip 20 extends along a respective edge of the panel 1, such that a spacing between the outer end of the diffuser strip 20 and the edge defines the window 24 for the beams that are emitted by the emitters 2. Alternatively, the diffuser 20 is in the form of pairs of diffuser strips 20 arranged with a gap, where the gap defines the window 24 for the beams that are emitted by the emitters 2. The reflector 22 is arranged on the front surface 5 to define a coherent frame around a central portion 40 of the panel 1, which frame provides the dual function of specularly reflecting light that impinges on it from inside the panel 1 and of hiding the incoupling structure, the emitters 2 and the detectors 3 from sight. The frame, which is made transparent in FIG. 12A to show the underlying components, covers the surface portion extending from the edge of the panel 1 to the dashed line. As explained in the foregoing, the frame may be replaced by spatially separate reflectors, e.g. of circular shape, which are arranged on the front surface 5 in optical correspondence with the respective emitter 2 and detector 3.

FIG. 12B is a top plan view of an alternative embodiment with an alternated arrangement of emitters 2 (open circles) and detectors 3 (squares) along the rim of the panel 1. Diffusers 20 in the form of spatially separate ring-shaped elements with a central window 24 are arranged on the rear surface 6. Like in FIG. 12A, a reflector 22 is arranged on the front surface 5 to define a coherent frame.

The detectors 3 may be optically coupled to the panel 1 in any suitable way. For example, in FIGS. 12A-12B, the detectors 3 may be attached to the rear surface by optical glue, gel, silicon, or the like. In FIGS. 12A-12B, the combination of diffuser 20 and reflector 22 will form an outcoupling structure similar to the incoupling structure. Most of the propagating light will strike the diffuser 20, and some of this light will be diffusively reflected onto the reflector 22, which in turn will reflect some of this light towards the window 24. The detectors 3 are arranged with an air gap or an intermediate spacer to the rear surface 6, in analogy with the foregoing incoupling embodiments. In the example of FIG. 2, the detectors 3 may be attached to the edge surface 10 of the panel 1. Generally, there are many other alternatives. For example, as disclosed in WO2012/105893, a sheet-like microstructure may be provided on the front or rear surface to couple light out of the panel. Further alternative outcoupling structures are disclosed in PCT/SE2013/050735, which was filed on Jun. 19, 2013, and application No. PCT/SE2013/050922, which was filed on Jul. 22, 2013, which are both incorporated herein by reference.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and the scope of the appended claims. 

1. A touch-sensitive apparatus, comprising: a light transmissive panel that defines a front surface and an opposite, rear surface; a plurality of light emitters optically coupled to the light transmissive panel and a plurality of light detectors optically coupled to the light transmissive panel, so as to define a grid of light propagation paths inside the light transmissive panel between pairs of light emitters and light detectors; wherein a structure for optically coupling one of the light emitters to the light transmissive panel comprises a first reflective element on the rear surface and a second reflective element on the front surface, the first reflective element being configured to be diffusively reflective to impinging light from inside the light transmissive panel, and the second reflective element being configured to be specularly reflective to impinging light from inside the light transmissive panel, and wherein said one light emitter is arranged to project a beam of light onto a transmissive surface portion on the rear surface, such that at least a portion of the beam of light enters the light transmissive panel through the transmissive surface portion, is specularly reflected against the second reflective element and impinges on the first reflective element from inside the light transmissive panel.
 2. The touch-sensitive apparatus of claim 1, wherein the transmissive surface portion is defined within the extent of the first reflective element.
 3. The touch-sensitive apparatus of claim 1, wherein the first reflective element is shaped as a ring of diffusively reflective material surrounding the transmissive surface portion.
 4. The touch-sensitive apparatus of claim 1, wherein the transmissive surface portion is formed by an uncovered portion of the rear surface between the first reflective element and a peripheral edge of the panel.
 5. The touch-sensitive apparatus of claim 1, wherein the structure is configured to define an origin for a subset of the light propagation paths (D), said subset of light propagation paths (D) extending in different directions from the structure across the light transmissive panel, and wherein the first reflective element is configured to have different extent in at least part of said different directions, and wherein the extent of the first reflective element in a subset of the different directions exceeds a limit extent W Um=2·t·tan(θ_(min)), with t being a thickness of the light transmissive panel and θ_(min) min being a minimum angle of incidence for light to propagate by internal reflections inside the light transmissive panel.
 6. The touch-sensitive apparatus of claim 5, wherein the first reflective element is elliptical.
 7. The touch-sensitive apparatus of claim 1, wherein the light transmissive panel comprises a peripheral edge surface that connects the front and rear surfaces, wherein a third reflective element is arranged on the peripheral edge surface adjacent to the first and second reflective elements, and wherein the third reflective element is configured to be diffusively reflective to impinging light from inside the light transmissive panel.
 8. The touch-sensitive apparatus of claim 7, wherein the first, second and third reflective elements define at least part of a reflective enclosure that extends from the first reflective element across the peripheral edge surface to the second reflective element.
 9. The touch-sensitive apparatus of claim 7, wherein said one light emitter is arranged to project the beam of light onto the transmissive surface portion such that a portion of the beam of light impinges on the third reflective element.
 10. The touch-sensitive apparatus of claim 1, wherein the first reflective element is a sheet-like element applied to or integrated in the rear surface of the light transmissive panel.
 11. The touch-sensitive apparatus of claim 1, wherein the first reflective element is non-transmissive to the beam of light.
 12. The touch-sensitive apparatus of claim 1, wherein the second reflective element is arranged opposite to the first reflective element.
 13. The touch-sensitive apparatus of claim 1, wherein the first reflective element is configured to exhibit at least 50% diffuse reflection, and preferably at least 90% diffuse reflection.
 14. The touch-sensitive apparatus of claim 1, wherein the first reflective element is a near-Lambertian diffuser.
 15. The touch-sensitive apparatus of claim 1, wherein the first reflective element is configured to promote emission of diffusively reflected light at angles that sustain light propagation by total internal reflection inside the light transmissive panel.
 16. The touch-sensitive apparatus of claim 1, wherein the light emitter is optically coupled to the transmissive surface portion by a light transmissive material with a refractive index that is adapted to the refractive index of the light transmissive panel at the transmissive surface portion.
 17. The touch-sensitive apparatus of claim 1, wherein the first reflective element is configured as an elongate strip, and the second reflective element is configured as an elongate strip which is arranged on the front surface to co-extend with the first reflective element, wherein the first and second reflective elements define a structure for coupling light from a number of light emitters into the light transmissive panel, the number of light emitters being located beneath the light transmissive panel dispersed along the first reflective element.
 18. The touch-sensitive apparatus of claim 17, wherein the first and second reflective elements define a respective frame structure around a center portion of the light transmissive panel.
 19. The touch-sensitive apparatus of claim 1, comprising a plurality of first reflective elements which are spatially separated and arranged on the rear surface along a rim of the light transmissive panel, wherein each of the first reflective elements is included in a structure for optically coupling a number of light emitters to the light transmissive panel, said number of light emitters being located beneath the light transmissive panel dispersed along the rim of the light transmissive panel.
 20. The touch-sensitive apparatus of claim 1, wherein the second reflective element is configured to block light that is visible to the human eye. 